Snap an empty to a bone's coordinate, quickest way?
I use the procedure described here to parent an Empty to a bone.  For example set the cursor to a location between a characters eyes; I then add an empty between the eyes (Empty.eyes.rig-dz1), parent that to a face bone to track (might also be able to add Copy Location constraint instead?).  I then have another character whose Eyes have an animated Copy Location (target==empty).  So that when it is enabled, eyes will look the other as the other moves.